Titanium dioxide of improved chalk resistance

ABSTRACT

Pigmentary titanium dioxide, especially titanium dioxide containing coreacted aluminum and silicon, is heat treated and then given a hydrous metal oxide coating, e.g., an alumina and silica coating. The resulting pigment has good chalk resistance and good retention of optical properties, such as gloss, color and tint efficiency.

United States Patent Dietz et a1.

Feb.8, 1972 [54] TITANIUM DIOXIDE OF IMPROVED CHALK RESISTANCE [72]Inventors: Albert Dietz, New Martinsville, W. Va.; Neil C. Goodspeed;Russell R. May, .Ir., both of Wadsworth, Ohio [73] Assignee: PPGIndustries, Inc., Pittsburgh, Pa.

[22] Filed: Feb. 28, 1969 21 Appl. No.: 803,448

[52] US. Cl ..l06/300, 106/308 B [51] Int. Cl [58] Field of Search..106/300, 3081, 308 B [56] References Cited UNITED STATES PATENTS2,559,638 7/1951 Krchmaetalm ....1 0 6 /3 00 2,689,781 9/1954Schaumann.... 3,146,119 8/1964 Ritter .....l06/300 3,468,689 9/1969 Lottet a]. 106/300 Primary Examiner-James E. Poer Assistant Examiner-H. M.S. Sneed Atlomey-Chisholm and Spencer ABSTRACT 7 Claims, No DrawingsTITANIUM DIOXIDE OF IMPROVED CHALK RESISTANCE BACKGROUND OF THEINVENTION Paint and lacquer films that are pigmented with regular(unmodified) titanium dioxide alone usuallyv exhibit a type of failureknown as chalking" in outdoor service, i.e., the film surface tends todisintegrate to a powdery chalk. This type of failure takes placecontinuously at the surface of the film and eventually results in totaldestruction of the film. Such failure is caused, in part, by the pigmentwhich, consequently, is said to possess poor chalk resistance.

Although many theories have been suggested to explain this. I type ofdisintegration, the'exact mechanism by which such failure occurs is notknown for certain. Since this type of disintegration occurs only onoutdoor exposure of the film, the theory advanced most often for thepoor weatherability of films pigmented with titanium dioxide is theaction of sunlight on the pigment.

There have been methods proposed for producing titanium dioxide pigmentsthat are resistant to outdoor weathering. One such method is toincorporate small amounts of conditioning agents such as antimonytrioxide, zinc oxide and rare earth compounds with the titanium oxidehydrolysate before calcination. Another proposed method is to coat theindividual particles of titanium dioxide after calcination with smallamounts of one or more hydrous oxides of such metals as aluminum,chromium, silicon, titanium, tin, thorium, cerium, zinc and zirconium.Such treatments often increase the pigments resistance to chalking,discoloration and fading to an appreciable extent but in some cases atsacrificing pigment color, brightness, tint efficiency and tintingstrength.

Although the above-suggested methods result in some improvement in theoutdoor weathering properties of titanium dioxide, there is a need forfurther improvement.

SUMMARY OF THE INVENTION It has now been discovered that titaniumdioxide having improved outdoor weathering properties, e.g., chalkresistance and gloss retention, can be produced by a combination ofphysical and chemical treatments. More particularly, such treatmentscomprise mildly heating pigmentary titanium dioxide prepared by vaporphase oxidation of titanium halide at temperatures of from about 500 toabout 750 C., and thereafter the thus heated titanium dioxide with atleast one hydrous metal oxide selected from the hydrous metal oxides ofaluminum, silicon, zirconium and titanium.

DETAILED DESCRIPTION Titanium dioxide pigment is produced commerciallyby at least two different manufacturing processes. One such process isknown as the sulfate or acid process. In the sulfate process, atitaniferous ore, such as ilmenite, is digested with sulfuric acid toform a digest cake. The cake is dissolved in an aqueous medium to form asulfate solution which, after clarification and concentration, ishydrolyzed to precipitate an insoluble titanium oxide hydrolysate. Thehydrolysate is filtered, washed and calcined at temperatures rangingbetween 800 and 1,200 C. or higher to develop the pigmentary propertiesof the pigment. Such pigment is often referred .to as sulfate pigment orcalcined titanium dioxide. The calcination can be conducted with orwithout additives, such as compounds of zinc. The additives are utilizedto reduce the effective calcination temperature and to promote thedevelopment of the rutile crystalline form. See, for example, U.S. Pat.Nos. 2,253,551, 3,062,673 and 3,330,798.

Another and more recent process developed for preparing pigmentarytitanium dioxide is that of the vapor phase oxidation of a titaniumhalide, such as titanium tetrahalide. Typically, this method involvesreacting vaporous titanium halide, e.g., titanium trichloride ortitanium tetrachloride, with oxygen in a reaction zone at temperaturesbetween about 800 and about l,200 C. Pigment prepared by theaforementioned process is often referred to as "chloride processpigment. The aforementioned vapor phase oxidation process is suitablydescribed in U.S. Pat. Nos. 3,068,113 and 3,214,284, which areincorporated herein, in toto, by reference.

In the chloride process," metal and nonmetal compounds, such as aluminumcompounds, zirconium compounds, silicon compounds and alkali metaland/or alkaline earth metal compounds are often added to the vapor phasereaction zone. The

presence and coreaction of such compounds aids in the production oftitanium dioxide having excellent pigmentary properties. The particularcompounds of aluminum, zirconium, silicon and the alkali and alkalineearth metals that can be used in the chloride process," as well as theirrespective amounts, are well known in the art. Reference is made to U.S.

Pat. No. 3,214,284, which has been incorporated herein, as an example ofthe state of the art.

The method of the present invention is especially applicable to rawpigmentary titanium dioxide prepared by vapor phase oxidation oftitanium halide, such as the tetrachloride, tetrabromide andtetraiodide, in the presence of aluminum and silicon compounds inquantities sufficient to provide from about 1 to about 2.5. weightpercent, preferably from about 1.5 to about 2.0 weight percent coreactedalumina, and from about 0.1 to about '110 weight percent, preferablyfrom about 0.3 to about 0.75 weight percent coreacted silica. Theaforementionedquantities of cobumed or cooxidized alumina and silica arecalculated as M 0 and SiO respectively based on the quantity of TiOproduced.

The term raw pigmentary titanium dioxide is intended to mean and includepigmentary titanium dioxide, especially rutile titanium dioxide, thathas had its basic pigmentary properties developed. Also included withinsaid term are pigments that have been physically treated, for example,by milling, grinding, hydroseparation, filtration, and degassingprocedures. Raw pigmentary titanium dioxide" is to be distinguished froma finished" titanium dioxide pigment which is defined herein as a rawpigment which has been given one or more inorganic and/or organiccoatings. Thus, a raw pigment is one which has had the basic chemicalcomposition of its surface substantially unaltered once the basicpigmentary properties have been developed; whereas, a finished" pigmentis a pigment which has had the chemical composition of its surfacealtered by the addition of a chemical coating to the surface of thepigment. An example of one such coating technique is found in U.S. Pat.No. 3,l46,l l9.

In accordance with the present invention, raw pigmentary titaniumdioxide is mildly heated at temperatures of from about 500 to about 750C., more preferably from 550 to about 700 C. and, thereafter, theresulting pigment is coated with at least one hydrous metal oxide ofmetals selected from the group consisting of aluminum, silicon,zirconium and titanium. Most preferably, the pigment is heated attemperatures of from about 6600 to about 650 C. The use of temperaturesbelow about 500 C. results in a pigment of poor photolytic stability.The use of temperatures greater than 750 C. results in degradation ofthe color of the pigment and poor photolytic stability.

The above-recited range of 500 to 750 C. is to be distinguished fromtemperatures at which titanium dioxide pigment is dried and temperaturesat which the pigment is calcined. Typical temperatures at which titaniumdioxide is dried to avoid degradation of the pigment range from about 60to about 300 C., usually range between and 220 C. Calcinationtemperatures typically range from about 800 to about l,200 C., and mostcommonly range from about 900 to about 1,000 C.

The pigment is heated at the temperatures recited hereinabove for a timesufficient to improve the resulting photolytic stability of the pigmentand the pigments durability, i.e., resistance to outdoor weathering.Generally, the

higher the heating temperature, the shorter the heating time required toobtain improved photol 'tic stability and vice ver- "sa. =Contemplatedare heating periods of from about 10 minutes to about 5 hours attemperatures of from 750 to about 500 C. respectively. Typically,heating is conducted for from about minutes to about l20 minutes,preferably from about 20 to about 30 minutes at temperatures of fromabout 600 to 650 C. Times of less than 10 minutes are usually inadequateto assure that all of the pigment is heat treated,

used, such heating times should be avoided at temperatures of greaterthan about 700 C. in order to avoid degradation of the pigmentsproperties.

Following heat treatment, it has been found expedient to mill the heattreated pigment, e.g., by fluid energy milling, to break up any pigmentagglomeration that has resulted from the heat treatment and to obtainoptimum dispersion in the aqueous slurry prepared with the heat treatedpigment.

The heat-treated pigment is then coated with at least one hydrous metaloxide of metals selected from the group consisting of aluminum, silicon,zirconium and titanium. Particularly contemplated are coatings ofhydrous alumina alone, hydrous silica alone, hydrous zirconia andsilica, hydrous silica and alumina, and hydrous alumina, silica andtitania. A combined coating of hydrous silica and alumina is preferred.Following application of the hydrous metal oxide to the pigment, thepigment is filtered, washed, dried and fluid energy milled. Theprocedure for the above-mentioned coating technique is artrecognized andis adequately described in publications and issued U.S. patents.Reference is made to US. Pat. No. 3,146,] 19 and copending US.application, Ser. No. 691,931, US. Pat. No. 3510,334, which areincorporated herein, in toto, by reference to illustrate theaforementioned coating technique. Only so much of said technique as isnecessary for the understanding of the present method without referenceto the aforementioned patent and patent application, therefore, will beincorporated herein. Other details can be found by reference to theaforementioned patent and patent application.

Typically, the coating procedure is performed in an aqueous medium attemperatures of between about 30 and about 100 C. and comprises addinghydrolyzable compound of silicon, aluminum, zirconium and titanium thatyield silica, alumina, zirconia, and titania hydrates, respectively,upon hydrolysis to an aqueous slurry of the titanium dioxide pigment.The aqueous slurry typically comprises between about 10 and about 30weight percent titanium dioxide and usually comprises about 20 weightpercent titanium dioxide.

The water in which the pigment is slurried should be such so as not toincorporate harmful contaminants into the pigment, i.e., the watershould be relatively pure, e.g., deionized or distilled water.

The hydrolyzable metal salts of aluminum, silicon, zirconium and/ortitanium are added to the slurry and hydrolyzed, if necessary, by theaddition of a suitable neutralizing agent, i.e., an acid or base,depending on the pH of the slurry andthe conditions under which saidmetal salts hydrolyze completely. If the slurry pH is alkaline, theslurry pH can be adjusted with an acidic agent, such as an inorganicacid (HCl H 80 H PO,, etc.) or acid yielding metal salts suchas-titanium tetrachloride and aluminum sulphate. If the slurry pH isacidic, the pH of the slurry can be adjusted with an alkaline agent. Anyconventional alkaline agent utilized in the pigment industry can beused. Preferably the alkaline material forms a halogen salt which issoluble in the liquid medium used to wash the flocculated pigment and/orvolatile under the conditions at which the pigment is dried. Typical ofthe alkaline agents that can be used, alone or in any combination,include sodium hydroxide, potassium hydroxide, lithium hydroxide,ammonium hydroxide, gaseous ammonia, potassium carbonate, sodiumcarbonate, potassium bicarbonate and sodium bicarbonate.

After the addition and hydrolysis of the desired hydrolyzable metalcompound(s), the pH of the pigment slurry is further adjusted, ifnecessary, to a level at which the pigment, now coated with hydrousmetal oxides, flocculates, i.e., between about 5 and about 8, usuallyabout 7.

The slurry typically is maintained at a temperature of from about 30 toabout 100 C., preferably to 85 C., for a period ranging from about 5minutes to about 20 hours, preferably from 1 to 3 hours, so as to digestthe slurry and insure proper setting of the hydrous metal oxide on thesurface of the pigment. Such digestion can be conducted after hydrolysisof the metal salts and/or after the slurry is brought into thefloc-forming range. The slurry, following such digestion, should have asubstantially neutral pH, i.e., from about 6.5 to about 7.5. The needfor a pigment with a pH near neutral is necessitated by the commercialrequirement that the finished pigment have a PH of approximately neutralso that it does not react with any oleoresinous vehicle into which it isincorporated.

Precipitation of more than one hydrous metal oxide onto the surface ofthe titanium dioxide pigment can be performed simultaneously or in anydesired sequence. Additional layers of hydrous metal oxide can beapplied to the pigment by repeating the aforementioned procedure, i.e.,reslurrying of the pigment, if necessary, addition of hydrolyzable metalsalts to the slurry, hydrolysis thereof, adjustment of the slurry pH anddigestion.

The aqueous slurry containing the coated, flocculated pigment is thenfiltered, washed to remove impurities such as salts of hydrolysis, driedand ground in a mill, e.g., a fluid energy mill such as a micronizer.

Titanium compound that can be used to coat the TiO,, pigment inaccordance with the present process are water soluble compounds oftitanium from which, upon hydrolysis, there can be precipitated titaniumoxide, a titania hydrate, e.g., Ti(OH TiO 'xH O, TiO(OH) or a titaniumhydrate condensate, e.g.,

wherein r is at least 2, or mixtures thereof. Examples of suitabletitanium compounds include: titanium tetrachloride, titaniumtetraiodide, titanium tetrabromide, titanium sulphate, titanyl sulfate,titanium esters, such as tetraethyl titanate, tetra-2-chloroethyltitanate, tetraphenoxy titanium and alkali metal titanates, e.g.,lithium, sodium and potassium titanates. Likewise, tetra-acyloxytitanium, for example, tetraacetyl titanium and tetrabutyric titaniumcan be employed. Titanium tetrachloride is economically preferred.

Aluminum compounds that can be used to coat the TiO pigment inaccordance with the present process are water soluble compounds ofaluminum from which, upon hydrolysis, there can be precipitated al(OH)an alumina hydrate, or an aluminum hydrate condensate, e.g.,

wherein n generally has a value of 2 or more. The free valences of therepeating unit are attached to hydrogen, alkoxy, aryloxy, acyloxy, AKOH)radicals or cross-linked with other materials of the same unit formula.

Examples of suitable aluminum compounds include: aluminum chloride,aluminum bromide, aluminum sulfate, alkyl and aryl aluminum such astriethyl aluminum trihexyl aluminum and triphenyl aluminum. Likewise,alkoxy and aryloxy aluminum such as triethoxy aluminum, 'tributoxyaluminum and triphenoxy aluminum can be employed. Acyloxy aluminum suchas triaeetyl aluminum can also be used. Aluminum chloride and aluminumsulfate are preferred.

Silicon compounds that can be used to coat the TiO pigment in accordancewith the present process are water soluble compounds of silicon fromwhich there can be precipitated a silica hydrate, e.g., Si(OH) SiO xH O,SiOy .xH O, SiO(OH) silicon oxide, a silicon hydrate condensate, e.g.,

wherein s has a value of at least 2 or mixtures thereof.

Examples of suitable silicon compounds include: silicic acid and alkalimetal silicates, e.g., sodium and potassium silicate, SiCl,,, SiBr,,,8H,, SiH,,, Si(OOCCl-l Si(OCH Si(NH Si(NH or other such hydrolyzablesilicon compounds. Preferably, the silicon compound is added to thepigment slur ry in the form of an aqueous solution.

Zirconium compounds that can be used to coat the TiO pigment inaccordance with the present process are water soluble compounds ofzirconium from which there can be precipitated a zirconia hydrate, e.g.,ZrO -xH O, zirconium oxide or a zirconium hydrate condensate. Examplesof suitable zirconium compounds include: ZIRCONIUM sulfate, zirconylchloride, zirconyl bromide, zirconyl iodide, zirconium fluoride andzirconyl nitrate.

The application of the hydrous titania, alumina, zirconia and silicacoatings can be chemically represented by the following unbalancedequations:

The titanium compound, preferably titanium tetrachloride, is added tothe pigment slurry in an amount sufficient to coat the titanium dioxidepigment with from 0.05 to 10 percent, preferably 0.1 to 3 percent, byweight titania, calculated as TiO based on the weight of the pigment.

The aluminum compound, preferably aluminum chloride or aluminum sulfate,is added to the pigment slurry in an amount sufficient to coat thetitanium dioxide pigment with from 0.05 to percent, preferably 0.5 to 5percent, by weight alumina, calculated at M 0 based on the weight of thepigment.

The silicon compound, preferably silicic acid or sodium silicate, isadded to the titanium dioxide pigment slurry in an amount sufficient tocoat the titanium dioxide pigment with from 0.01 to l0 percent,preferably from 0.4 to 4 percent, by weight silica, calculated as SiObased on the weight of the pigment.

The zirconium compound, preferably zirconium sulfate or zirconylchloride. is added to the pigment slurry in an amount suflieienl to coatthe titanium dioxide pigment with from 0.05 to l5 percent, preferably0.5 to 5 percent, by weight zirconia. calculated as ZrO based on theweight of the pigment.

The above quantities of silicon, aluminum zirconium and titaniumcompounds added to the pigment slurry are typical of those amountsconventionally used. Quantities greater or less than those recited canbe employed. Of particular utility are coatings of hydrous alumina andhydrous silica wherein from about l to 3 weight percent of each, basedon TiO is used. Typically, the percentage ratio of hydrous alumina tohydrous silica (weight percent based on TiO is 1:3 to 3:l. The totalamount of hydrous metal oxide coating placed on the titanium dioxidepigment typically ranges from about 2 to about 20 weight percent,preferably 3 to 15 weight percent, based on the weight of the pigment.

After filtering the pigment slurry, impurities such as salts are removedby washing of the filtered flocs with an extractant such as water,alcohols, ethers, ketones, or mixtures of same.

After washing, the pigment is dried at temperatures of from 60 to 300C., preferably 100 to 220 C., e.g., by means ofan oven, belt dryer, orspray dryer, and then milled, e.g., in a fluid energy mill.

In a typical embodiment of the present invention, titanium dioxideprepared by vapor phase oxidation of titanium tetrachloride is heated ina rotary kiln at 650 C. for about 30 minutes. The resulting pigment isfluid energy milled and then slurried in deionized water'to give aslurry of about 20 weight percent solids. The pH of the resulting slurryis adjusted to between about 9 and 10 and the slurry hydroclassified ina series of Dorrclone classifiers. Sodium silicate and aluminum sulfatesolutions sufficient to yield, respectively, about 1 weight percentsilica and about 3 weight percent alumina, based on TiO are added to theoverflow from the Dorrclone classifier and the resulting slurry heatedfrom ambient temperature to about C. The slurry is neutralized to a pHof about 7 with anhydrous ammonia and sodium carbonate and digested forabout an hour at 85 C. The alumina-silica coated TiO pigment isrecovered by filtration. The filter cake is washed with deionized water,dried at about 1 10 C., and fluid energy milled.

Evaluation of the pigments produced in accordance with the processdescribed herein, such as in the succeeding examples, can be performedutilizing conventional pigment tests, as well as by outdoor exposure ofpaint panels prepared with such pigments.

The tinting strength and tint tones of pigments can be determined bymeans of A.S.T.M. Method D32226 which is found in the 1949 Book ofA.S.T.M. Standards, published by the American Society for TestingMaterials, Philadelphia 3, Pa.

The tint efficiency of a pigment, which is a measure of the ability of awhite pigment to resist tinting by a colored pigment, can be determinedby the reflectometry method described in A.S.T.M. Method D-2745-68T.This method was orginally described by Mitton and Jacobsen in theJournal of Paint Technology and Engineering, (Official Digest, Volume34, pp. 704-715,.luly, i962).

The percent gloss retained of an exposed paint panel can be measured bya Hunter Glossmeter. A procedure for this method is found in A.S.T.M.Method D52362T. A high gloss retention percentage is an indication ofincreased durability for a pigment.

Photoreactivity of the pigment can be determined by the use of theMandelie Acid Test. This test depends on the photochemical reduction oftitanium dioxide to a lower oxide with the simultaneous oxidation of aliquid medium such as aqueous solutions of stannous chloride, glycerol,aqueous solutions of mandelic acid, octyl alcohol, and mineral oil.Although glycerol produces the fastest reaction, mandelic acid is oftenemployed because of operational advantages such as workability of thepaste. This test method is described by A. E. Jacobsen in IndustrialEngineering Chemistry, Vol. 4l,p. 524, (1949).

Basically, this test comprises mulling about 10 grams of the samplepigment into a soft paste with the liquid medium such as glycerol or a0.5 molar solution of mandelic acid in water. The paste is placed on aglass plate 5 5 l).04 inches, covered by another plate of the same sizeand pressed out to a surface area of 4 inches in diameter. The edges ofthe plate are bound with %-inch cellulose adhesive tape to preventdrying of the paste. An initial reflectance is made with a tristimulusreflectometer and recorded. For exposure, the glass slide of the samplepigment and a glass slide of a similarly prepared standard pigment areplaced on the outer edge of a rotating table below a source ofultraviolet light such as a sunlamp. Additional readings are made atsuitable intervals of time and the rate of discoloration of theexperimental pigment relative to the standard determined. As a result ofthese reading, the experimental pigment is given a rating, based on thestandard, of from I to 10. In this scale, 1 is the least photoreactiveand the most photoreactive. A pigment yielding a rating of from I to 3is usually considered to have good resistance to photoreactivity.

Titanium dioxide is a well-known commercial white pighydrous oxidecoating described in preparation A.

Each of the aforementioned prepared pigments (preparations A and B) wastested for l photoreactivity by the mandelic acid test, and (2) tintefficiency. Portions of the aforesaid pigments, as well as threecommercially available titanium dioxide pigments, were each incorporatedinto both a thermoset acrylic vehicle and a melamine alkyd vehicle.

Paint panels prepared with the thermoset acrylic vehicle compositionwere exposed at 5 horizontal, Florida south for l8 months and the and 60gloss retained measured by a Hunter Glossmeter at 12 and 18 months.Paint panels from the melamine alkyd vehicle were exposed at 5horizontal, Medina (Ohio) south for 24 months and the 20 gloss retainedmeasured by a Hunter Glossmeter. The results of the above tests aretabulated in Table I.

TABLE I Pigment preparation l pigment X a Commercial pigment Y bCommercial pigment Z c reactivity Melamine-alkyd Thermoset acrylicvehicle vehicle Ohio Florida exposure (series 061) exposure (series 062)Gloss readings Gloss readings Tint mos. mos.

Photo- Mandelic acid test 24 month & Automotive enamel grade pigment.

b Durable grade sulfate process T102 pigment. 0 Durable grade sulfateprocess 'IiOz pigment *Average value for samples of the pigment.

ment and is used in the paint, coatings and paper industry.

The present process is more particularly described in the followingexamples which are intended as illustrative only since numerousmodifications and variations therein will be apparent to those skilledin the art.

EXAMPLE I PREPARATION A To an aqueous slurry of the base pigment (about20 weight percent solids) at ambient temperatures (about 25 C.) wasadded sufficient sodium silicate and aluminum sulfate to yield about 1.0weight percent hydrous silica (calculated as SiO and about 3.2 weightpercent hydrous alumina (calculated as A1 0 respectively, based on T10The resulting slurry was heated to about 85 C. and digested at thattemperature for about 1 hour. Thereafter, the slurry was neutralizedwith anhydrous ammonia to a neutral pH and the slurry digested to insureproper setting of the hydrous silica and alumina coating. The coatedpigment was recovered by filtration, washed with deionized water, driedat a temperature of about 1 l0 C. and fluid energy milled in a Trostmill.

PREPARATION B from a source ditierent than (D).

The data of Table I show that paint panels prepared with pigmentprocessed in accordance with the present invention (preparation B) havea definite advantage in 20 and 60 gloss retention and tint efficiencyfor the vehicle systems tested when compared with two commercial durablegrade sulfate process TiO pigments.

EXAMPLE II Titanium dioxide prepared by vapor phase oxidation oftitanium tetrachloride in the presence of aluminum chloride and silicontetrachloride was processed further in accordance with the proceduresset forth hereinafter. The base pigment had a tinting strength value ofabout 1680, a tone of Neutral, an oil absorption of about l8 andcontained about 1.7 weight percent coreacted alumina and about 0.3weight percent coreacted silica, both based on TiO PREPARATION A To anaqueous slurry of the base pigment (about 20 weight percent solids) atambient temperatures (about 25 C.) was added sufficient sodium silicateand aluminum sulfate to yield about 1.0 weight percent hydrous silica(calculated as SiO and about 3.2 weight percent hydrous alumina(calculated as A1 0 respectively, based on TiO The resulting slurry washeated to about C. and digested at that temperature for about 1 hour.Thereafter, the slurry was neutralized with anhydrous ammonia to aneutral pH and the slurry digested to insure proper setting of thehydrous silica and alumina coating. The coated pigment was recovered byfiltration, washed with deionized water, dried at a temperature of about1 10 C. and fluid energy milled in a Trost mill.

PREPARATION B Base pigment was heat treated for about 1 hour at about600 C. in a muffle furnace. The heat-treated pigment was fluid energymilled in a Trost mill and then given the same hydrous oxide coatingdescribed in preparation A.

Each of the aforementioned prepared pigments (preparations A and B) wastested for l) photoreactivity by the mandelic acid test and (2) tintefficiency. Portions of the aforesaid pigments, as well as the threecommercial titanium dioxide pigments recited in Example I and Table l,were each incorporated into a thermoplastic acrylic vehicle and anair-DRY alkyd vehicle. Paint panels prepared with the thermoplastic Theresults of 'l'ahle lll show that a paint panel prepared with pigmentprocessed in accordance with the present invention (preparation A) isequal or better in 60 gloss retention in the vehicle system tested thantwo commercial durable grade acrylic vehicle were exposed at horizontal,Medina (Ohio) 5 chloride process TiO pigments. south for months and the60 gloss retained measured by a whi there am above described a number fifi Hunter Glossmeter. Palm pa ls p ep d from the y bodiments of thepresent invention, it is obviously possible to alkyd vehicle wereexposed at 5 horizontal, Florida south and produce other embodiments andvarious equivalent modificathe 60 gloss retained after 18 monthsexposure m a r tions thereof without departing from the spirit of theinven- The results of the above tests are tabulated in Table ll. tion,

. TABLE II Thermoplastic Air-dry alkyd acrylic Ohio Photo Floridaexposure exposure runetlvlty 'llnt (HOI'il H (ml) (nurlott 114) Mttlltlillt ulllill!" gloss rmttlluus till tzlontt tt lttlllltttl l'l rntmitprnpnrntlon ueld test. liUllU-y ttl months in months A 2 00 31 11 B 2 0342 28 Commercial pigment X0) 92. 5 25 43 Commercial pigment Y( )t "87. 520 33 Commercial pigment Z() 87. 5 19 Automotive enamel grade pigment.Durable grade sulfate process TiO2 pigment.

Average value for samples of the pigment.

The data of Table II show that the paint panel prepared with pigmentprocessed in accordance with the present invention (preparation B) has adefinite advantage to the two durable grade pigments shown in tintefficiency and in 60 gloss retention in an air-dry alkyd vehicle.

EXAMPLE lll Titanium dioxide prepared by vapor phase oxidation oftitanium tetrachloride in the presence of aluminum and silicon wasprocessed further in accordance with the procedure set forthhereinafter. The bass pigments optical properties and coreacted silicaand alumina content were comparable to those recited for the basepigments of Examples 1 and II.

PREPARATION A Base pigment was heat treated for about 1 hour at about600 C. in a mufile furnace. The heat treated pigment was fluid energymilled in a Trost mill and then given a hydrous alumina-hydrous silicacoating in the same manner as described in preparation A of Example I.

A portion of the pigment prepared in accordance with Preparation A, andportions of three commercially available titanium dioxide pigments, wereeach incorporated into an airdry alkyd vehicle. Paint panels preparedwith the air-dry alkyd vehicle compositions were exposed at 5horizontal, Florida south. The 60 gloss retained by these panels after 8months exposure was read by a Hunter Glossmeter. The results of thereadings are tabulated in Table lll.

TABLE III Commercial Pigment U Commercial Pigment V"' Commercial PigmentW (a) durable grade chloride process TiO,

(h) durable grade chloride process TiO from a source other than (a) (c)gloss enamel TiO pigment set forth the general nature and specificembodiments of the present invention, what is claimed is particularlypointed out in the appended claims.

1. in the method of treating raw pigmentary titanium dioxicle containingfrom about i to about 2.5 weight percent cooxidized alumina and fromabout 0.3 to about 0.75 weight percent cooxidized silica by coating saidpigment with at least one member selected from the group consisting ofthe hydrous metal oxides of aluminum, silicon, zirconium and titanium,the improvement which comprises increasing the photolytic stability ofsaid pigment by heating the raw pigment at from about 500 to about 750C. for from about 5 hours to ten minutes, respectively, before coatingsaid pigment with said hydrous metal oxide.

2. in the method of treating raw pigmentary titanium dioxide containingfrom about i to about 2.5 weight percent cooxidized alumina and fromabout 0.3 to about 0.75 weight percent cooxidized silica by coating saidpigment with hydrous alumina and hydrous silica, the improvement whichcomprises increasing the photolytic stability of said pigment by heatingsaid raw pigment at from about 600 to about 650 C. for from about 20 toabout 30 minutes before coating said pigment with said hydrous metaloxide.

3. A method according to claim 2 wherein the total amount of hydrousmetal oxide coating ranges from about 2 to about 20 weight percent,based on titanium dioxidev 4. A method according to claim 1 wherein saidraw pigment is heated at from about 600 to about 650 C. for from about20 to about 30 minutes.

5. A method according to claim 1 wherein the total amount of hydrousmetal oxide coating ranges from about 2 to about 20 weight percent,based on titanium dioxide.

6. Pigmentary titanium dioxide prepared by the method of claim 1.

7. Pigmentary titanium dioxide prepared by the method of claim 2.

2. In the method of treating raw pigmentary titanium dioxide containingfrom about 1 to about 2.5 weight percent cooxidized alumina and fromabout 0.3 to about 0.75 weight percent cooxidized silica by coating saidpigment with hydrous alumina and hydrous silica, the improvement whichcomprises increasing the photolytic stability of said pigment by heatingsaid raw pigment at from about 600* to about 650* C. for from about 20to about 30 minutes before coating said pigment with said hydrous metaloxide.
 3. A method according to claim 2 wherein the total amount ofhydrous metal oxide coating ranges from about 2 to about 20 weightpercent, based on titanium dioxide.
 4. A method according to claim 1wherein said raw pigment is heated at from about 600* to about 650* C.for from about 20 to about 30 minutes.
 5. A method according to claim 1wherein the total amount of hydrous metal oxide coating ranges fromabout 2 to about 20 weight percent, based on titanium dioxide. 6.Pigmentary titanium dioxide prepared by the method of claim 7.Pigmentary titanium dioxide prepared by the method of claim 2.